Die bond touch down detector

ABSTRACT

A die bond touch down detector (20) includes a strain gauge circuit (22) having a plurality of strain gauges (12) formed into a wheatstone bridge configuration. The strain gauges (12) are mounted on a die bond head (10) and measure and detect deformations of the die bond head (10). The strain gauge circuit (22) generates a differential output signal in response to deformations detected by the strain gauges (12). The differential output signal is amplified, filtered, and converted to digital format for processing by a microcontroller (36). The microcontroller (36) performs calibration, display formatting, and touch down signal generation. The die bond touch down detector (20) provides real time monitorization, automatic calibration, and real force applied information for the die bond head (10).

This is a continuation of application Ser. No. 08/405,332, filed Mar.16, 1995 and now U.S. Pat. No. 5,608,172.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to semiconductor fabricationprocesses and more particularly to a die bond touch down detector.

BACKGROUND OF THE INVENTION

Close detection of touch down positioning and real force applied on thebond heads of die attach bonders, wire bonders, or other equipment is animportant requirement for calibration and real time monitorization ofthe bond head applied force. Previous die bond systems use piezoelectricsensors that induce a piezoelectric voltage in performing touch downdetection. These piezoelectric sensors have limited sensitivity, requiretemperature compensation, noise producing, and are relatively costly.Typically, Z axis height calibrations have been executed manually,resulting in subjective determinations that cause errors in the diebonding process from one operator to another. Existing die bond systemsare of poor quality, costly, have calibrating problems, and translatethe position of and not the real force applied to the bond head. Thereal force applied to the bond head, if determined, is indirectlymeasured by the difference between the control and the feedback of agalvanometer. Therefore, it is desirable to have a die bond detectorthat can perform close detection of touch down positioning and determinethe real force applied by the bond head during real time monitoringoperations.

SUMMARY OF THE INVENTION

From the foregoing, it may be appreciated that a need has arisen for atouch down detector that can perform real time monitorization of thereal force applied by a bond head. A need has also arisen for a touchdown detector that can perform automatic calibration functions toachieve better performance in the die bonding process.

In accordance with the present invention, a die bond touch down detectorand method are provided that substantially eliminate or reducedisadvantages and problems associated with conventional die bondingprocesses.

According to an embodiment of the present invention, there is provided adie bond touch down detector that includes a die bond head and aplurality of strain gauges in contact with the die bond head. The straingauges are capable of detecting and processing deformations of the diebond head.

The present invention provides various technical advantages overexisting die bonding processes. For example, one technical advantage isin the use of strain gauges to detect deformations on the die bond head.Another technical advantage is in directly detecting the real forceapplied by the die bond head. Yet another technical advantage is toautomatically calibrate the touch down detecting circuitry on the diebond head. Still another technical advantage is in performing real timemonitoring of die bond head position and force applied. Other technicaladvantages are readily apparent to one skilled in the art from thefollowing figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like parts, in which:

FIG. 1 illustrates a simplified diagram of a die bond head;

FIG. 2 illustrates a simplified schematic diagram of a die bond touchdown detector; and

FIG. 3 illustrates a graph of a calibration method performed by the diebond touch down detector.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram of die bond head 10. Attached to die bond head 10are a plurality of strain gauges 12. Strain gauges 12 are placed atparticular points on die bond head 10 in order to precisely measuredeformations of die bond head 10 and determine the mechanical stressplaced on die bond head 10. Strain gauges 12 are attached to the surfaceof die bond head 10 and obtain punctual measurements of the deformationsof die bond head 10. Strain gauges 12 are passive sensors that converttheir own deformations, in response to the deformations of die bond head10, into resistance variations.

In the embodiment of FIG. 1, four strain gauges are affixed to die bondhead 10. Two strain gauges 12 are located in the upper side of die bondhead 10 and two strain gauges 12 are located in the lower side of diebond head 10. Four strain gauges 12 are used to increase the sensitivityof the detection system. Strain gauges 12 are fixed at a place on diebond head 10 where there is maximum torque. With this location, accuratedetection of die bond head touch down and force can be achieved.Preferable characteristics for strain gauges 12 include a gauge lengthof 6 millimeters, a gauge resistance of 120 +/-0.3 Ohms, and a gaugefactor of 2.13.

FIG. 2 is a block diagram of a die bond touch down detector 20. Die bondtouch down detector 20 includes a strain gauge circuit 22 that can beadjusted and balanced by a voltage reference circuit 24 and an offsetcircuit 26, respectively. Strain gauge circuit 22 provides an input toan amplifier 28 that is adjusted by a gain circuit 30. The output ofamplifier 28 is filtered by filter circuit 32 and converted into digitalform by analog-to-digital converter 34. The digital signal fromanalog-to-digital converter 34 is processed by a microcontroller 36 thatdrives a display device 38 and performs calibrations in response toparameters retained by a memory 40. Communication betweenmicrocontroller 36 and the positioning equipment for die bond head 10may be performed through an RS232 device 42. Microcontroller 36 alsoprovides a reference value to a digital-to-analog converter 44 in orderto generate a touch down detection trigger signal through a comparator46. A calibration circuit 48 generates the interrupts to microcontroller36 in order to calibrate the system.

To acquire information from strain gauges 12, a signal conditionercircuit is used. This circuit associates one voltage variation to theresistance variation that is obtained by strain gauges 12 when they aresubmitted to mechanical stress as applied to die bond head 10. Straingauge circuit 22 uses a bridge as its signal conditioning circuit.Bridges are double potentiometric circuits with a differential outputvoltage signal that provides less noise and are less voltage supplydrift sensitive than simple potentiometric signal conditioner circuits.Strain gauge circuit 22 uses one complete Wheatstone Bridgeconfiguration. Each bridge arm contains one strain gauge 12 placed in amanner where each half bridge has two strain gauges that have oppositeresistance variations. The equivalent resistance variation is alsoopposite from one half bridge to the other half bridge. The followingequations show the relationship between the resistance variations ofstrain gauges 12 and the output differential voltage of strain gaugecircuit 22 with the respective strain gauge deformation.

    R.sub.1 =R+ΔR=R(1+kε)                        (1)

    R.sub.2 =R-ΔR=R(1-kε)                        (2)

    R.sub.3 =R-ΔR=R(1-kε)                        (3)

    R.sub.4 =R+ΔR=R(1+kε)                        (4) ##EQU1##

Voltage reference circuit 24 supplies a stabilized voltage to straingauge circuit 22. Voltage reference circuit 24 may employ an adjustableprecision shunt regulator. The use of a voltage regulator is importantbecause the differential output of strain gauge circuit 22 depends onthe voltage supply. Therefore, the output of strain gauge circuit 22 isas accurate as the stability of its voltage supply.

Offset circuit 26 adjusts the balance of strain gauge circuit 22. Offsetcircuit 26 compensates the deviation of strain gauge circuit 22 when itis at rest. The adjustment provided by offset circuit 26 is performedwhen strain gauge circuit 22 is at rest until the output differentialtension of strain gauge circuit 22 is zero.

The differential output signal of strain gauge circuit 22 is received atamplifier 28. Amplifier 28 is used to amplify the very smalldifferential signals and reject all common mode voltage from thedifferential output signal of strain gauge circuit 22. The gain ofamplifier 28 is adjusted by gain circuit 30. Gain adjustments shouldagree with system calibrations performed by microcontroller 36.

After amplification, the differential output signal passes through a lowpass filter within filter circuit 32. Filter circuit 32 may be a lowpass second order Biquad active filter with fs=100 Hertz. Filter circuit32 eliminates oscillations generated by vibrations of the step motor ofthe positioning equipment that positions die bond head 10. Filtercircuit 32 also eliminates oscillations due to vibrations of arm springsattached to die bond head 10. The filtered output signal is thenconverted to digital form by analog-to-digital converter 34 in order tobe processed by microcontroller 36.

Microcontroller 36 performs all the control functions for die bond touchdown detector 20. The basic functions performed by microcontroller 36are to carry out the mathematical computations to implement a linearregression method for calibration of the system, to transform theanalog-to-digital converter values into display values using calibrationparameters, and to generate a touch down reference signal to send to thepositioning equipment for die bond head 10. The use of microcontroller36 enables real time control of calibration and filters undesirablenoise and accurately detects touch down moment through digital signalprocessing.

System calibration is performed by microcontroller 36 in response toscale calibration parameters within memory 40. Memory 40 is preferably anon-volatile memory that retains its data when the system shuts down andeliminates the need to calibrate the system each time it is reset.

Microcontroller 36, through display device 38, provides thevisualization in the decimal base of the direct output fromanalog-to-digital converter 34. Display device 38 may be composed offour seven segment displays driven by one display driver.

Microcontroller 36 also generates a reference value in determining touchdown detection. The reference value is converted to analog form bydigital-to-analog converter 44 and then compared to the output fromamplifier 28 in comparator 46. Comparator 46 generates a touch downtrigger signal for the positioning equipment of die bond head 10.Comparator 46 is a level detector with a reference tension equal to theoutput reference of microcontroller 36 of the calibrated scale and withthe input voltage connected to the output of instrumentation amplifier28.

Microcontroller 36 performs a linear regression calibration method thatestablishes two referential points. These two points are obtained bysubmitting die bond head 10 to two different weights. The first weightis defined as zero grammes with a die bond head 10 without weight andstrain gauge circuit 22 at rest. The second weight is defined as 100% ofthe scale, meaning that the values displayed are in percentage of theweight defined as 100% of the scale. The linear regression method may beconsidered as a function specified by the two referential points thattransforms input values into pressure, or calibrated force, values.

FIG. 3 shows a graph of the linear regression function line. With thisgraphic representation, it is easy to conclude that if the gain ofamplifier 28 is too high, the 100% calibration point may not be possiblewith any weight because it may cause the analog-to-digital converter 34to be out of range. If the gain of amplifier 28 is too low, it may causea low resolution calibration. These opposing factors should be takeninto account to establish the gain of amplifier 28 to obtain the optimumcalibration line.

The real time force and touch down information provided by strain gauges12 and the automatic calibration performed by microcontroller 36 providean improved bonding process that avoids the temperature compensationtechniques of piezoelectric sensors and the subjective manualcalibration that induces error between different operators and does nottranslate the real force applied by the die bond head 10. Die bond touchdown detector 20 makes it possible to control the force exerted by diebond head 10 during the bonding process. In this way, the system avoidstechnical assistance during lot set up due to differences in the diethickness from lot to lot. Die bond touch down detector 20 may also beused for dynamic measuring of damping characteristics of moving parts.

In summary, a die bond touch down detector uses a plurality of straingauges to measure and detect physical deformations of the geometry ofthe die bond head upon encountering an obstacle. The die bond touch downdetector includes a microcontroller that performs automatic calibrationof the positioning equipment in order to accurately detect occurrence oftouch down of the die bond head. The die bond touch down detectorperforms real time monitorization of the real force applied to the diebond head.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a die bond touch down detector and method thatsatisfy the advantages set forth above. Although the preferredembodiment has been described in detail, it should be understood thatvarious changes, substitutions, and alterations can be made herein. Forexample, though specific connections have been shown, certain circuitrymay be coupled together through intermediate circuitry instead of bydirect connections as shown in the FIGUREs. Other examples are readilyascertainable by one skilled in the art and can be made withoutdeparting from the spirit and scope of the present invention as definedby the following claims.

What is claimed is:
 1. A die touch bond detector, comprisinga die bondhead; a plurality of strain gauges located on said die bond head, saidplurality of strain gauges connected to form a wheatstone bridge,wherein each half of said wheatstone bridge contains at least two ofsaid plurality of strain gauges having opposite resistance variationsand the equivalent resistance variation is opposite from one half ofsaid wheatstone bridge to the other half.
 2. The die bond touch downdetector of claim 1, wherein said plurality of strain gauges performreal time monitoring of said die bond head.
 3. The die bond touch downdetector of claim 1, wherein said plurality of strain gauges detect realforce applied by said die bond head.
 4. A die bond touch down detector,comprising:a die bond head having an upper and lower surface; aplurality of upper strain gauges located on said upper surface; and aplurality of lower strain gauges located on said lower surface, saidpluralities of upper and lower strain gauges connected to form awheatstone bridge, wherein each half of said wheatstone bridge containsat least one of said upper strain gauges and at least one of said lowerstrain gauges, and wherein one of said upper strain gauges from one halfbridge is connected to one of said lower strain gauges of the other halfresulting in opposite equivalent resistance variations between thehalves.